The present invention relates in general to alternating current power supplies. More particularly, the present invention relates to improving voltage regulation in alternating current electric power systems.
It is known that electric power systems which have large inherent system reactance and/or supply highly reactive loads are characterized by poor voltage regulation, i.e., substantial change in the magnitude of load voltage as load current fluctuates. In a typical inductive circuit, voltage magnitude and power factor both decrease as load current increases. To improve voltage regulation, power transformers are commonly provided with load tap changers to counteract the tendency of voltage magnitude to change with change in load current. FIG. 1 is a schematic diagram of a prior art transformer 10 incorporating a conventional load tap changer 20. Also shown are terminals 12, 14, and 16.
Traditional voltage regulation for power transformers includes either no-load or under-load taps that connect directly to either primary or secondary windings of the supply transformer with switching means to change the tap connections for the desired voltage range as required by the resistance of the load. This approach requires complex design arrangements and causes several undesired effects such as reduction in the ability to withstand transient by the transformer windings, uneven short circuit forces during faults, reduced dielectric performance, higher overall costs, and larger dimensions of the transformers.
Voltage regulation using transformer taps typically uses off-load taps in the main transformer winding, a regulating winding, an off-load tap changer, an on-load tap changer, or auxiliary transformers. Taps add considerable cost to a standard transformer design: about a 4 to 10 percent increase for off-load taps and about a 20 to 30 percent increase for on-load taps. It is also more difficult and time consuming to manufacture such a system, typically taking about 30 to 40 percent more hours to manufacture a transformer with taps. Moreover, taps distort the leakage flux in the windings, and lead to higher eddy losses and circulating currents, higher localized heating and hot spots, and higher short-circuit forces. Furthermore, there is higher transient stress at the tap regions which affects the thermal performance and could cause dielectric failure. Moreover, tap changers are unreliable, with over 40 percent of field failures being attributed to tap changer failures.
The present invention is directed to the voltage regulation of power transformers by the use of a separate, removable, detachably coupled device external to the main transformer that can be attached to a main transformer unit when voltage regulation is desired. The device is connected to the three neutrals of the main transformer and can comprise: (a) a bank of three capacitors connected in wye, (b) a bank of three medium voltage (MV) or low voltage (XV) transformers, (c) one three-phase low voltage or medium voltage transformer, or (d) a combination of a XV/MV transformer and a capacitor bank.